Thallium (Tl) is one of the most toxic of the heavy metals. Its continued use as a rodenticide in many developing countries and its increasing use in an expanding number of new technologies raise concerns about exposure risk to animals and humans. Because Tl and potassium (K) have the same charge and similar ionic radii, Tl follows K distribution pathways and alters and number of K-dependent processes. Possible toxic mechanisms of Tl include ligand formation with protein sulfhydryl groups, inhibition of cellular respiration, interaction with riboflavin and riboflavin-based cofactors, and disruption of calcium homeostasis. The principal clinical features of thallotoxicosis are gastroenteritis, peripheral neuropathy of unknown etiology, and alopecia. The presence of elevated Tl levels in the urine or other biologic materials confirms the diagnosis of Tl poisoning. Treatment with prussian blue (or activated charcoal) will interrupt the enterohepatic cycling of Tl, thus enhancing fecal elimination of the metal. Forced diuresis with potassium loading will increase the renal clearance of Tl, but should be used cautiously because neurologic and cardiovascular symptom may be exacerbated. If recognized and treated early, Tl poisoning carries a favorable prognosis for full recovery.